1. Field of the Invention
The present invention relates to photodegradable and thermostable compositions based on homopolymers and copolymers of vinyl or vinylidene monomers, such as polyethylene, polypropylene, polyisobutene, polystyrene, polyvinyl chloride and ethylenepropylene copolymers.
2. The Prior Art
It is quite well known that vinyl and vinylidene polymers are widely used as packaging materials in the form of films or in the form of containers of different types. It is also well known that the stability of such packagings with the passage of time is rather high, so much so as to cause serious environmental pollution problems due to the lack of an efficient system for the gathering and destruction of such materials. The environmental pollution caused by the residues of these packaging materials may be avoided by the use of degradable materials that decay under atmospheric conditions or under biological attack.
In particular, the degradability of vinyl or vinylidene polymers under the photochemical action of sunlight may usually be enhanced by mixing such polymers with photosensitizing substances. Photosensitizing substances which are in common use at present, are salts of the transition metals and compounds containing particular chromophore groups such as carbonyls or double bonds. The presence of such substances in the polymeric composition accelerates the photooxidation of the polymeric chain, with consequent degradation thereof. This degradation may in fact proceed to the point of reducing the manufactured article to a fine dust, and possibly up to the point of reducing the molecular weight of the polymeric material to values that are low enough for subsequent biological degradation.
In general, it is known that a photodegrading action will be exerted on the above-noted polymers by transition metal compounds. Specifically, such action will be exerted by all transition metal compounds that are soluble in the polymer and in which the transition metal has an atomic number between 21 and 30, 40 and 47 and 57 and 79 inclusive, wherein the binder or anion that is chemically bound to the metal, does not itself possess intrinsic photostabilizing properties. As examples of such compounds, mention may be made of the stearates, naphthenates, laurates, palmitates, oleates, sulphonates, phenolates, phosphonates, phosphites, oxides, acetyl acetonates, dibenzoylacetonates, alkylthiocarbamates, complexes with hydroxybenzophenone, cyclopentadiene, mono- and polyamines, oximes, ketones and thioketones, hydrazines, azo-compounds, etc. of copper, titanium, cobalt, iron, nickel, manganese, chromium, niobium, molybdenum, cerium, tungsten, etc. The compounds of the above-described type and their photodegrading properties with regard to vinyl and vinylidene polymers, are both quite well known in the art. They are described in, for example, German Patent Application No. 2,136,704. The majority of the above transition metal compounds which act as photodegrading agents, have, however, a deleterious action on the thermal stability of the vinyl and vinylidene polymers.
On the other hand, since the processing of these polymers and their transformation into manufactured articles take place at high temperatures, the presence of highly thermodegradable substances such as the transition metal compounds, causes degradation of the polymer in the processing stage, as a result of which there are obtained products possessing rather poor physical-mechanical properties.
It is, of course, known that one may hinder or slow down the thermal degradation of a polymer by adding to the polymer suitable substances which act as thermal stabilizers. For this purpose, there are commonly used aromatic amines, such as phenyl-1-naphthylamine and N'N'-diphenyl-p-phenylenediamine, certain aryl phosphites, such as triphenylphosphite, phenols such as 2,2'-methylenbis(4-methyl-6-t-butylphenol), 2,6-di-t-butylparacresol, 4,4'-thiobis(2-methyl-6-t-butylphenol), and other organic compounds of different types.
In general, it may happen, however, that the addition of a thermal stabilizer will reduce the photodegrading action of the transition metal compound. Thus, in most instances it becomes necessary to effect a compromise between thermal stability and photodegradability, using particular ratios between the transition metal compound and the thermal-stabilizing agent.
It is an object of the present invention to overcome these problems of the prior art, and in accordance with the invention as will be hereinafter described, this has been achieved.